Transport in Plants | 植物运输

📚 Transport in Plants | 植物运输

Plants are complex living organisms that require a continuous supply of water, mineral ions and sugars to survive and grow. Unlike animals, plants do not have a heart or a circulatory system. Instead, they use two highly specialised vascular tissues – xylem and phloem – to transport substances over long distances. This article explains the structure, function and adaptations of these transport systems, as required by the Edexcel IGCSE Biology specification.

植物是复杂的生物体，需要持续不断的水分、矿物质离子和糖类才能生存与生长。与动物不同，植物没有心脏或循环系统。它们依靠两种高度特化的维管组织——木质部和韧皮部——来完成远距离运输。本文将依据 Edexcel IGCSE 生物考纲，详细讲解这两套运输系统的结构、功能及其适应性。

1. Why Do Plants Need Transport Systems? | 植物为什么需要运输系统？

Multicellular plants are large in size, with a small surface area to volume ratio. Diffusion alone is too slow to move water and dissolved substances from the roots to the leaves, or sugars from the leaves to other parts of the plant. Therefore, plants have evolved dedicated transport systems: xylem and phloem, which together form the vascular bundles.

多细胞植物体积较大，其表面积与体积之比相对较小。单靠扩散作用，不足以将水分和溶解物质从根部运送到叶片，也无法将糖类从叶片运输到其他部位。因此，植物演化出了专门的运输系统：木质部和韧皮部，它们共同构成维管束。

2. The Two Transport Tissues: Xylem and Phloem | 两种运输组织：木质部与韧皮部

The xylem carries water and dissolved mineral ions from the roots upwards to the rest of the plant. The phloem carries sugars (mainly sucrose) and amino acids from the leaves to all other parts, including roots, fruits and storage organs. Xylem and phloem are arranged together in vascular bundles, which run through the roots, stem and leaves.

木质部将水分和溶解的矿物质离子从根部向上输送到植物体各处；韧皮部则将糖类（主要是蔗糖）和氨基酸从叶片运送到根、果实、贮藏器官等所有部位。木质部和韧皮部在维管束中相伴排列，贯穿根、茎和叶。

3. Structure of Xylem and Phloem | 木质部与韧皮部的结构

The xylem is composed of dead cells, which are hollow and lined with lignin. Lignin is a tough, waterproof substance that strengthens the walls and prevents the vessels from collapsing. Because the living contents are absent, xylem vessels form continuous tubes with little resistance. The phloem is made of living cells, including sieve tube elements and companion cells. Sieve tubes have perforated end walls called sieve plates, allowing sap to flow between cells. Companion cells have many mitochondria to supply energy for transport.

木质部由死细胞构成，细胞中空，内壁沉积木质素。木质素是一种坚硬、防水的物质，可增强细胞壁并防止导管塌陷。由于无原生质体存在，木质部导管形成连续通畅的管道，阻力很小。韧皮部由活细胞组成，包括筛管元件和伴胞。筛管的端壁上有筛板，有利于汁液在细胞间流动；伴胞含有大量线粒体，为运输提供能量。

4. Water Absorption by Root Hairs | 根毛对水分的吸收

Water enters the plant through root hair cells, which are thin-walled extensions of epidermal cells in the root. These cell extensions greatly increase the surface area for absorption. The soil solution is usually more dilute than the cytoplasm and vacuole of root hair cells, so water enters by osmosis – the net movement of water from a region of higher water potential to a lower water potential, across a partially permeable membrane.

水分通过根毛细胞进入植物体。根毛是根表皮细胞向外延伸形成的细长突起，细胞壁薄，大大增加了吸收面积。通常情况下，土壤溶液比根毛细胞的细胞质和液泡更稀，因此水分依靠渗透作用进入根毛——即水分子从水势较高区域跨过部分透性膜向水势较低区域净移动。

Osmosis: water moves from high water potential to low water potential | 渗透：水从高水势移向低水势

5. The Transpiration Stream | 蒸腾流

Once inside the root, water moves between cells or through the cell walls until it reaches the xylem. The xylem distributes water and minerals to all parts of the plant. The continuous upward movement of water from the roots to the leaves is called the transpiration stream. This is driven by transpiration – the loss of water vapour from the leaves – which creates tension (negative pressure) in the xylem, pulling water up. Cohesion between water molecules helps maintain an unbroken column of water.

水分进入根部后，通过细胞间隙或细胞壁运输至木质部。木质部将水分和矿物质分配到植物各部位。由根到叶的持续向上水流称为蒸腾流。蒸腾作用——叶片向外界散失水蒸气——会产生的张力（负压）驱动水流上升。水分子之间的内聚力有助于维持连续不断的水柱。

6. Transpiration and the Factors Affecting It | 蒸腾作用及其影响因素

Transpiration is actually evaporation from the surfaces of leaf cells followed by diffusion of water vapour through stomata into the surrounding air. The rate of transpiration is affected by several environmental factors:

蒸腾作用实质上是叶肉细胞表面的水分蒸发，随后水蒸气通过气孔扩散到周围空气中。蒸腾速率受以下环境因素影响：

Light intensity: in bright light, stomata open wider for photosynthesis, so more water vapour escapes.

光照强度：光照强时，气孔为光合作用而开得更大，水蒸气逸出更多。
Temperature: higher temperature increases the rate of evaporation and diffusion of water vapour.

温度：温度升高会加快水分蒸发和水蒸气扩散速率。
Humidity: when the air is humid, the concentration gradient of water vapour is low, so transpiration slows down.

湿度：空气潮湿时，水蒸气浓度梯度小，蒸腾作用减慢。
Wind speed: moving air removes water vapour, maintaining a steep concentration gradient and increasing transpiration.

风速：流动空气带走水蒸气，保持较大的浓度梯度，从而加快蒸腾作用。

7. Translocation in the Phloem | 韧皮部中的转运作用

Translocation is the transport of soluble organic substances (assimilates) such as sucrose and amino acids in the phloem. It occurs in both directions, from sources to sinks. A source is a part of the plant that produces or releases assimilates, e.g. mature leaves during photosynthesis or storage organs during germination. A sink is a part that uses or stores them, e.g. growing roots, developing fruits or bulbs.

转运作用是指韧皮部中将可溶性有机物（同化物），如蔗糖和氨基酸，进行的运输。转运方向为从源到库，可以是双向的。源是产生或释放同化物的部位，例如进行光合作用的成熟叶片或发芽时的贮藏器官；库是利用或储存同化物的部位，如生长中的根、发育中的果实或鳞茎。

The movement of phloem sap is explained by the pressure flow hypothesis. Sucrose is actively loaded into sieve tubes at the source, lowering water potential, so water enters by osmosis. This increases hydrostatic pressure in the phloem at the source. At the sink, sucrose is removed, raising water potential, so water leaves, reducing pressure. The pressure difference drives the flow of sap.

韧皮部汁液的流动可用压力流假说解释。在源处，蔗糖被主动装载入筛管，降低水势，水分便通过渗透作用进入筛管，使源端静水压升高。在库端，蔗糖被取用或卸出，水势升高，水分渗出，压力下降。这种压力差驱动汁液流动。

8. Experiments to Investigate Transport | 研究运输过程的实验

A common way to show that water travels through the xylem is to place a celery stalk or a white flower in water containing a dye. After a few hours, the coloured solution is found in the vascular bundles, proving that xylem is the water-carrying tissue. A potometer can be used to measure the rate of water uptake by a leafy shoot under different conditions, which is an indirect measure of the transpiration rate.

证明水分经由木质部运输的常用方法是，将芹菜茎或白色花朵放入含有染料的水中。几小时后，可见染色溶液出现在维管束中，从而证明木质部是输送水分的组织。用蒸腾计（potometer）可测量植物枝条在不同条件下的吸水速率，这可以间接反映蒸腾速率。

Potometer: distance moved of air bubble ÷ time gives rate of water uptake | 蒸腾计：气泡移动距离 ÷ 时间 = 吸水速率

9. Adaptations to Reduce Water Loss | 减少水分散失的适应性特征

Terrestrial plants often face water shortage. Many have adaptations to reduce transpiration: a waxy cuticle on the leaf surface reduces water loss through the epidermis; stomata are often located mainly on the lower epidermis, away from direct sunlight; in many plants stomata close when the plant is short of water. Some plants, such as marram grass and cacti, have rolled leaves or spines to reduce surface area and decrease water loss.

陆地植物常常面临缺水问题。许多植物具有减少蒸腾作用的适应性特征：叶片表面的蜡质角质层可减少通过表皮的水分散失；气孔多数分布于下表皮，从而避开直射光；许多植物在缺水时会使气孔关闭。部分植物，如滨草和仙人掌，具有卷曲叶片或刺，以减少表面积并降低水分丢失。

10. Comparing Xylem and Phloem | 木质部与韧皮部的比较

Feature	Xylem \| 木质部	Phloem \| 韧皮部
Main transported substance	Water and mineral ions	Sucrose and amino acids
Direction of flow	Root to leaves (upward)	Source to sink (can be up or down)
Living or dead cells	Dead (empty tubes)	Living (sieve tubes + companion cells)
Cell wall	Thickened with lignin	Thin cellulosic wall, sieve plates
Energy requirement	No direct energy needed for water flow	Companion cells supply ATP for active loading

11. Common Exam Mistakes | 常见考试易错点

Students often confuse “transpiration” with “translocation”. Remember: transpiration is water loss as vapour; translocation is sugar transport in phloem. Another common error is stating that water rises in xylem due to “suction” from the root. In fact, the pull caused by transpiration, not root pressure, is the main mechanism. Also, do not say that phloem carries “food” only; be specific about sucrose and amino acids.

学生常把“蒸腾作用”和“转运作用”混淆。请牢记：蒸腾作用是水蒸气散失；转运作用是糖类在韧皮部中的运输。另一个常见错误是认为水分在木质部中上升是因为根的“抽吸力”。实际上，蒸腾产生的拉力才是主要机制，而不是根压。另外，不要只说韧皮部运输“食物”，应具体说明是蔗糖和氨基酸。

12. Summary | 总结

In summary, plants rely on the xylem to transport water and minerals upward, and the phloem to transport sugars and amino acids from sources to sinks. The structural adaptations of these tissues are tightly linked to their functions. Understanding the roles of transpiration and translocation, and the factors that influence them, is essential for solving IGCSE Biology questions on this topic.

总之，植物依靠木质部将水分和矿物质向上运输，依靠韧皮部将糖类和氨基酸从源运输到库。这两类组织的结构与功能密切相关。理解蒸腾作用和转运作用及其影响因素，是解答 IGCSE 生物这类题目的关键。

Published by TutorHao | Biology Revision Series | aleveler.com

更多咨询请联系16621398022（同微信）

Transport in Plants | 植物运输